1. Field of the Invention
The present invention relates to a color image display device (active matrix color image display device) in which information such as a picture image is displayed by switching elements and pixels arranged in a matrix form, particularly to a digital system driving method and an image display device using the same, and to an electronic equipment.
2. Description of the Related Art
In recent years, a technique of fabricating a semiconductor device including a semiconductor thin film formed on an inexpensive glass substrate, for example, a thin film transistor (TFT), has been rapidly developed. The reason is that the demand for an active matrix image display device has increased.
The active matrix image display device includes an active matrix liquid crystal image display device using liquid crystal for a display element, an EL display device using an electro luminescence (EL) element, and the like. Hereinafter, as a typical example of the active matrix image display device, the active matrix liquid crystal display device will be described.
As shown in FIG. 30, the active matrix liquid crystal display device includes a source signal line driving circuit 101, a gate signal line driving circuit 102, and a pixel array portion 103 disposed in a matrix form. The source signal line driving circuit 101 samples an inputted picture signal in synchronization with a timing signal such as a clock signal and writes the data into respective source signal lines 104. The gate signal line driving circuit 102 sequentially selects gate signal lines 105 in synchronization with a timing signal such as a clock signal and controls the on and off of a TFT (pixel TFT) 106 as a switching element in each of pixels of the pixel array portion 103. By this, data written in the respective source signal lines 104 are sequentially written in the respective pixels.
Although a driving system of the source signal line driving circuit includes an analog system and a digital system, attention has been paid to the digital system active matrix liquid crystal display device in which high definition and high speed driving can be achieved.
A conventional digital system source signal line driving circuit is shown in FIG. 31. In FIG. 31, reference numeral 201 designates a shift register portion which is constituted by a shift register basic circuit 202 including flipflop circuits and the like. When a start pulse SP is inputted to the shift register portion 201, sampling pulses are sequentially transmitted to first latch circuits 203 (LAT 1) in synchronization with a clock signal CLK.
In synchronization with the sampling pulses from the shift register portion, the first latch circuits 203 (LAT 1) sequentially store n-bit (n is a natural number) digital picture signals supplied from data bus lines (DATA-R, DATA-G, DATA-B).
After signals for one horizontal pixels are written in the LAT 1 portion, the digital picture signals held in the respective first latch circuits 203 (LAT 1) are transferred all together to second latch circuits 204 (LAT 2) in synchronization with a latch pulse supplied from a latch signal bus line (LP).
When the digital picture signals are held in the second latch circuits 204 (LAT 2), the start pulse (SP) is again inputted, and digital picture signals for pixels of a next line are newly written in the LAT 1 portion. In the meantime, the digital picture signals for the pixels of the former line are stored in the LAT 2 portion, and analog picture signals corresponding to the digital picture signals are written in respective source signal lines by digital/analog signal conversion circuits (hereinafter referred to as D/A conversion circuits) 205 (D/A). In FIG. 31, reference characters Vref-R, Vref-G and Vref-B respectively designate gray-scale power supply lines connected to the D/A conversion circuits 205 corresponding to respective colors of R (red), G (green) and B (blue). Reference characters SL1, SL2, . . . designate numbered source signal lines, R, G, B written below SL1 and the like designate red, green and blue, respectively, and it is assumed that the display device can produce a color display with a single plate.
Each of the respective D/A conversion circuits 205 shown in FIG. 31 is connected to one source signal line, and the analog picture signal is written in the one source signal line. However, in the case where a liquid crystal display device of high resolution and high definition is fabricated, forming the same number of D/A conversion circuits, each occupying a large area, as the source signal lines is an obstacle to the miniaturization of the liquid crystal display device which is desired in recent years, and a method of driving a plurality of source signal lines by one D/A conversion circuit is proposed in Japanese Patent Application Laid-open No. Hei. 11-167373.
FIG. 32 shows a structural example of a source signal line driving circuit for driving four source signal lines by one D/A conversion circuit. As is understood from comparison with FIG. 31, a parallel/serial conversion circuit 301 (P/S conversion circuit), a source signal line selecting circuit 302, and a selecting signal (SS) inputted to those are newly added in FIG. 32. In spite of the fact that such circuits are added, if writing of signals in four source signal lines can be made by one D/A conversion circuit, the effect that the number of necessary D/A conversion circuits can be made ¼ of the original number is great, and it becomes possible to decrease the occupied area of the source signal line driving circuit.
In FIG. 31, the gray-scale power supply lines of three independent systems for RGB are supplied to the source signal line driving circuits. However, differently from FIG. 31, a gray-scale power supply line of only one system is supplied to the source signal line driving circuit shown in FIG. 32. In general, when a power supply voltage of the gray-scale power supply line is given, the output voltage range of the D/A conversion circuit is uniquely determined. Thus, in the source signal line driving circuit of FIG. 32 to which the gray-scale power supply line of one system is supplied, the ranges of voltages written in the respective source signal lines become same and irrespective for RGB.
The dependency of a luminance ratio of a liquid crystal display device on a voltage applied to a liquid crystal is not quite the same for the respect colors of RGB, and it is different according to the colors as an example shown in FIG. 33. In this example, a voltage value where the luminance ratio has the minimum value is VR, (<) VG, (<) VB for the respective colors of RGB and is different from one another. Thus, when a voltage is applied to a liquid crystal, in order to prevent the monotonicity of gray-scale display from being lost, the maximum voltage which can be applied to the liquid crystal become VR, VG or VB for the respective colors of RGB. However, in the case where a gray-scale power supply line of only one system is supplied as shown in FIG. 32, the range of voltage which can be applied to the liquid crystal becomes uniform and irrespective for RGB as described above, so that the maximum voltage which can be applied becomes VR for the liquid crystal having the luminance ratio—voltage characteristics of FIG. 33. At this time, there occur problems that G and B do not become sufficiently dark states, and the contrast becomes low, and further, an expression property for accurate color becomes poor.
From the above reason, as shown in FIG. 31, it is desirable to enable an applied voltage to the liquid crystal to be controlled independently for RGB by such a method as to provide gray-scale power supply lines of three systems independently for RGB.
However, in the case where a plurality of source signal lines are driven by one D/A conversion circuit in the method of providing gray-scale power supply lines of three systems, the number of the gray-scale power supply lines is increased, and a switch for switching connection between one of those gray-scale power supply lines and the D/A conversion circuit becomes necessary. These cause new problems such as an increase in the number of external input pins, and an increase in the occupied area of the driving circuit by a region for wiring of the gray-scale power supply lines, the added switch, and the like. Under such circumstance, the merit of driving a plurality of source signal lines by one D/A conversion circuit and decreasing the occupied area of the driving circuit is lost.